A NOx selective reduction catalyst is used in some cases in order to purify NOx contained in the exhaust gas discharged or exhausted from an internal combustion engine. In relation to the NOx selective reduction catalyst, it is known that the reduction efficiency of NOx varies depending on the ratio between NO and NO2 contained in the exhaust gas. Accordingly, a technique is disclosed, in which the amount of urea as a reducing agent supplied to the exhaust gas is controlled based on the ratio between NO and NO2 contained in the exhaust gas (see, for example, Patent Document 1). In the case of this technique, an oxidation catalyst is provided on the upstream side from a selective catalytic reduction NOx catalyst in order that the ratio between NO and NO2 in the exhaust gas is about 1:1. Further, the supply of urea to the exhaust gas is controlled based on the concerning ratio, and thus it is aimed to perform the efficient NOx purification.
When the oxidation catalyst and the NOx selective reduction catalyst are successively arranged in the flow of the exhaust gas as described above, if the oxidation catalyst is in a state of being poisoned with SOx contained in the exhaust gas, then the oxidizing ability is lowered. Therefore, it is difficult to provide a desired value of the ratio between NO and NO2 contained in the exhaust gas allowed to flow into the NOx selective reduction catalyst. In view of the above, a technique is disclosed, in which a NOx adsorbing catalyst is provided on the more upstream side of the oxidation catalyst, SOx contained in the exhaust gas is adsorbed thereby, and thus the oxidizing ability of the oxidation catalyst is suppressed from being lowered (see, for example, Patent Document 2).